help with a dc power supply

[bonhome]

hello, i have a 5VDC power supply ,with 220 to 9 V AC transformer ,two diodes , a filter , and a 7805 regulator.
and i want to know how to protect my load from reverse polarity,and overvoltage.
thank you.

 

[Medtronic]

dear my friend
with 7805 regulator no problem with over voltage , and you can put a diode at the output to protect from polarity

best regards

 

[alexan_e]

A diode in the output of 7805 will not do anything, it will just provide a lower voltage because of the diode forward voltage drop.
To protect a load you have to connect the diode in the INPUT OF EACH LOAD so it can prevent the voltage to go through if the polarity is reversed

You also have to be aware that any diode in the input of your device will introduce a voltage drop
starting from about 0.2v up to 0.5v (schottky) or more for a normal diode.

Alex

 

[Medtronic]

my friend you can put a diode at the ground and at the output of the regulator without any drop.

thanks

---------- Post added at 13:00 ---------- Previous post was at 12:59 ----------

i mean one at the ground terminal and one at the output

 

[alexan_e]

my friend you can put a diode at the ground and at the output of the regulator without any drop.
i mean one at the ground terminal and one at the output

if you have invented a diode that doesn't have a voltage drop i would b very interested...

In any case if you connect a diode in the ground side and one in the positive side (in series) these don't provide any protection,
the 7805 is not going to invert the positive and ground side before the diodes so these will be always on no matter how you will connect your load.

The diode should go to the positive input of the load so if the positive input of the load (including the input diode) goes to the grd this diode will be reverse biased and will not conduct.

Alex

 

[bonhome]

thanks that was helpful

 

[sanjubluerock3]

i have attached the pcb layout and schematic of variable dc using lm317T range 1.5v to 32v dc use eagle to open the files

 

[Gandalf_Sr]

There is actually a chip by Maxim that enables an ideal diode MAX16914, MAX16915. These ICs control 2 FETs so that the voltage lost is I x Rds-on, realistically a few millivolts with the right FET.